Soil fertility management in pasture small-plot trials: potential pitfalls

Small-plot cutting trials are of particular importance in research relating to intensive pastures. Undetected changes in soil fertility during the course of experimentation may detract from the validity of results in trials of this kind. Information from field trials conducted in KwaZulu-Natal during the past two decades are used to illustrate the potential impact of soil fertility aspects in small-plot cutting trials. A major potential pitfall is the underestimation of potassium (K) depletion accompanying the removal of herbage at harvesting. In an Italian ryegrass trial 518kg K ha-1 was removed in harvested material in a single season. Failure to compensate (by fertilizer topdressings) for removals of K may result in growth, particularly in the more favourable treatments, being inadvertently restricted. Soil acidification during the course of experimentation is a second factor which may threaten the usefulness of data from cutting trials. Acidification is promoted by the application of relatively high rates of N fertilizers; in this context, variable acidification where N fertilization is a factor in trials warrants particular attention. Removals, immobilization and leaching of phosphorus and sulphur as well as inherent soil heterogeneity on the trial site are additional important considerations in small-plot trials. The only effective means of dealing with these problems is through the testing of soil samples drawn from individual treatments at suitably regular intervals.Keywords: Nutrient removals, soil acidity, soil heterogeneity, soil potassium, soil testing.African Journal of Range & Forage Science 16 2&3): 101–10